He then tweeted an hour later: "The Great State of Arizona, where I just had a massive rally (amazing people), has a very weak and ineffective Senator, Jeff Flake. Sad!"

In an interview with Jake Tapper on CNN's "State of the Union," Flake, a vocal Trump detractor, said he "simply can't" support Trump.

"I would not vote for Hillary Clinton, and as of now, I would still not vote for Donald Trump," Flake said.

"I just know that I would like to vote for Donald Trump. It's not comfortable to not support your nominee," he added. "But given the positions he's taken, and the tone and tenor of his campaign, I simply can't."

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Politicians who refuse to support Donald Trump

Mitt Romney has been critical of Trump's rhetoric. REUTERS/Jim Urquhart

Senator John Thune (R-SD) addresses delegates during the third session of the Republican National Convention in Tampa, Florida, August 29, 2012. REUTERS/Mike Segar (UNITED STATES - Tags: POLITICS ELECTIONS)

Jeb Bush, former Governor of Florida and 2016 Republican presidential candidate, was one of Donald Trump's primary targets during the primary season.

Daniel Acker/Bloomberg via Getty Images

Former Republican presidential candidate and Ohio Governor John Kasich stayed in the primary longer than most other candidates, and notably refused to appear at the GOP convention in the same arena with Trump, attending other events instead. (Photo by Alex Wong/Getty Images)

Sen. Lindsey Graham (R-SC), a close friend to Sen. John McCain, has been a vocal critic of Trump's. (Photo by Drew Angerer/Getty Images)

UPDATE: Although he didn't endorse Trump during the 2016 convention, Ted Cruz eventually changed his mind, saying in September he'd vote for the GOP nominee (Photo by Ida Mae Astute/ABC via Getty Images)

U.S. Senator Kelly Ayotte (R-NH) addresses the second session of the Republican National Convention in Tampa, Florida August 28, 2012. REUTERS/Mike Segar (UNITED STATES - Tags: POLITICS ELECTIONS)

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Flake also suggested that Trump is in danger of losing Arizona to Democratic candidate Hillary Clinton, a belief he's held publicly since June. Arizona has voted Republican in every election except one since 1952. Bill Clinton in 1996 was the only Democrat to carry the state in that span.

"Arizona should still be a red state. But Donald Trump, with the rhetoric that he's under and the characterizations of, you know, many of the state's population, have put the state in play," Flake said.